Time Division Multiplexing : Block Diagram, Working, Differences & Its Applications

A medium can carry only a single signal at any second in time. To transmit multiple signals to transmit a medium, the medium has to be separated by providing every signal a segment of the whole bandwidth. This can be possible by using a multiplexing technique. Multiplexing is a technique is used to combine various signals into a single signal using a shared medium. There are different types of multiplexing techniques like TDM, FDM, CDMA & WDM which are utilized in data transmission systems. This article discusses an overview of one of the types of multiplexing techniques like time division multiplexing which is also known as TDM.

What is Time Division Multiplexing?

Time-division multiplexing or TDM definition is; a multiplexing technique that is used to transmit two or above streaming digital signals above a common channel. In this type of multiplexing technique, incoming signals are separated into equivalent fixed-length time slots. Once multiplexing is done, these signals are sent over a shared medium & after de-multiplexing, they are reassembled into their original format.

Block Diagram of Time Division Multiplexing

The time division multiplexing block diagram is shown below which uses both the sections of the transmitter and receiver. For data transmission, the multiplexing technique which efficiently utilizes the whole channel is sometimes called PAM/TDM because; a TDM system utilizes a PAM. So in this modulation technique, every pulse holds some short time period by allowing maximal usage of the channel.

In the above TDM block diagram, there is the number of LPFs at the beginning of the system based on the no. of data inputs. Basically, these low-pass filters are anti-aliasing filters that remove the aliasing of the data i/p signal. After that, the LPF’s output is given to the commutator. According to the commutator’s rotation, the data inputs samples are gathered through it. Here, the rate of revolution of the commutator is ‘fs’ therefore it denotes the system’s sampling frequency.

Assume that we have ‘n’ data inputs, and then according to the revolution one after the other, these data inputs will get multiplexed and transmitted above the common channel. At the receiver end of the system, a decommutator is used which is synchronized at the transmitting end by the commutator. So this de-commutator l at the receiving end divides the time division multiplexed signal.

In the above system, the commutator & de-commutator should have the same rotating speed in order to have precise demultiplexing of the signal at the end of the receiver. Based on the revolution performed through the decommutator, the samples are gathered through the LPF & the actual data input at the receiver is recovered.

Let the maximum frequency of signal ‘fm’ & the sampling frequency ‘fs’ then

fs ≥ 2fm

Therefore, the duration of time in between succeeding samples is given as,

Ts = 1/fs

If we consider that there are ‘N’ input channels, then a single sample is gathered from each of the ‘N’ samples. Therefore, every interval will give us ‘N’ samples & the spacing among the two can be written as Ts/N.

We know that basically pulse frequency is the number of pulses for each second which is given as
Pulse frequency = 1/spacing between two samples

= 1/ Ts/N =.N/Ts

We know that Ts = 1/fs, the above equation will become as;

= N/1/fs = Nfs.

For a time division multiplexing signal, the pulse for each second is the rate of signaling that is denoted with ‘r’. So,

r = Nfs

How Does Time Division Multiplexing Work?

Time-division multiplexing method works by putting several data streams within a single signal by dividing the signal into various segments, where each segment has a very short duration. Every individual data stream at the receiving end is reassembled depending on the timing.

In the following TDM diagram, when the three sources A, B & C wants to send data through a common medium, the signal from these three sources can be separated into various frames where every frame has its fixed time slot.

In the above TDM system, three units from every source are taken into consideration that forms the actual signal jointly.

A frame is collected with a single unit of each source that is transmitted at a time. When these units are different completely from each other, then preventable signal mixing chances can be removed. Once a frame gets transmitted above a specific time slot, then the second frame utilizes a similar channel to get transmitted & further this process is repeated until the transmission is completed.

Types of Time Division Multiplexing

There are two types of time division multiplexing; synchronous TDM and asynchronous TDM.

Synchronous TDM

The input is synchronous time division multiplexing is simply connected to a frame. In TDM, if there are ‘n’ connections, then the frame can be separated into ‘n’ time slots. So, each slot is simply allocated to every input line. In this method, the sampling rate is familiar to all signals, and thus similar clock input is given. The mux assigns the same slot to every device at all times.

The advantages of synchronous TDM mainly include; order being maintained and no addressing data is necessary. The disadvantages of synchronous TDM mainly include; it needs a high bit rate and if there is no input signal at a single channel since a fixed time slot is allocated to every channel, then the time slot for that specific channel does not hold any data & there is bandwidth wastage.

Asynchronous TDM

Asynchronous TDM is also known as Statistical TDM which is a type of TDM where the o/p frame gathers information from the input frame till it is filled but not leaving an unfilled slot like in Synchronous TDM. In this type of multiplexing, we have to include the address of particular data within the slot that is being transmitted to the output frame. This type of TDM is very efficient because the capacity of the channel is completely used & improves the efficiency of bandwidth.

The advantages of asynchronous TDM mainly include; its circuitry is not complex, low capacity communication link is used, there is no severe crosstalk problem, no intermediation distortion and for each channel, the complete channel bandwidth is used. The disadvantages of asynchronous TDM mainly include; it needs a buffer, frame sizes are different and address data is required.

Difference B/W Time Division Multiplexing Vs Time Division Multiple Access

The difference between TDM and TDMA are discussed below.

Advantages and Disadvantages

The advantages of time division multiplexing include the following.

• The circuit design of TDM is simple.
• TDM uses the total bandwidth of the channel for signal transmission.
• In TDM, the intermediation distortion issue is not there.
• TDM systems are very flexible compared to FDM.
• For every channel, the complete available channel bandwidth is used.
• Sometimes, pulse overlapping can cause crosstalk however it can be decreased using guard time.
• In this multiplexing, unwanted signal transmission between communication channels takes place rarely.

The disadvantages of time division multiplexing include the following.

• Both the transmitting & receiving sections should be synchronized properly to have right signal transmission & reception.
• TDM is complex to implement.
• As compared to FDM, this multiplexing has lower latency.
• TDM systems require addressing the data & the buffer.
• The channels of this multiplexing may get exhausted because of slow narrowband fading.
• In TDM, synchronization is very significant.
• In a TDM, a buffer & address information are necessary.

Applications/Uses

The applications of time division multiplexing are discussed below.

• TDM is utilized in Integrated Services Digital Network telephone lines.
• This multiplexing is applicable in public switched telephone networks (PSTN) and SONET (Synchronous Optical Networking).
• TDM is applicable in telephone systems.
• TDM is utilized in wireline telephone lines.
• Earlier, this multiplexing technique is used in the telegraph.
• TDM is used in cellular radios, satellite access systems, and digital audio mixing systems.
• TDM is the most common technique used in fiber optic communication/optical data transmission systems.
• TDM is used for analog & digital signals where a number of channels with less speed are simply multiplexed into high-speed channels are utilized for transmission.
• It is used in cellular radio, digital communication & satellite communication system.

Thus, this is an overview of time division multiplexing or TDM which is used for transmitting multiple signals above the same shared medium by simply allocating a limited time interval to every signal. Generally, this type of multiplexing is used through digital systems that send or receive digital bandpass or digital signals which are carried over analog carriers & utilized by optical transmission systems like SDH (Synchronous Digital Hierarchy) & PDH (Plesiochronous Digital Hierarchy). Here is a question for you, what is FDM?